The narrator is Dupin's friend and roommate. Like Dupin and G—, the narrator is in all three of the Dupin tales. We're just going to say it: he seems really unimportant. He hardly says anything; he never appears to leave the library; and he doesn't even act as a Watson-type-sounding board for Dupin.
But you could still think of him as the story's most important character. He's not only telling the story; he's shaping it, and he's shaping it with a clear bias toward Dupin. (Check out "Narrator Point-of-View" for more about that.)
More important, all his information is second- and third-hand. Other than a brief glimpse of the letter, the narrator sees nothing for himself. But he sure does hear a lot. Not that he shares everything he hears, though. He never describes the letter; he never explains what the letter contains; and he never spills the exact identities of the people involved (although presumably he knows). But do these omissions make him more trustworthy—or less?